Conventional automatic transmissions used in vehicles typically include a torque converter, a powertrain realized through a multi-stage gearshift mechanism that is connected to the torque converter, and a hydraulic control system that selectively operates one of a plurality of operational elements of the powertrain according to a driving state of the vehicle.
The powertrain is provided through a compound planetary gearset that combines at least two simple planetary gearsets to control shifters into a required shift range, and a plurality of friction elements. The friction elements are selectively engaged by hydraulic pressure supplied from the hydraulic control system to achieve shifting.
The powertrain may be configured differently depending on the manufacturer, but most automatic transmissions now used are four-speed automatic transmissions. However, much attention is being given to the development of five-speed automatic transmissions, which are being introduced into some vehicles. Four or five speeds are nevertheless inadequate in many instances in that the difference in gear ratios between the different speeds is large, which leads to an undesirable jerking motion during shifting, and high fuel consumption.